ABSTRACT: Gametes of marine broadcast spawners are highly susceptible to the threats of ocean warming and acidification. Here, we explore the main and interacting effects of temperature and pH changes on sperm motility and fertilization rates in the mussel Mytilus galloprovincialis. Additionally, we determine how temperature and pH interact to influence the motility of aging sperm. We show that the interactive effects of temperature (18°C or 24°C) and pH (ranging from 7.6 to 8.0) on sperm motility depend on the time that sperm spend in these conditions. Specifically, sperm linearity was influenced by a temperature × pH interaction when measured after a relatively short exposure to the treatment conditions, while main effects of temperature and pH (but no interaction) on sperm motility became apparent only after prolonged exposure (2 h) to the treatments. Despite the interactive effects of temperature and pH on sperm motility, these factors had independent effects on fertilization rates, which were significantly higher at the ambient ocean pH level and at the elevated temperature. This study highlights the importance of considering the combined effects of predicted ocean changes on sperm motility and fertilization rates, and cautions against using only sperm motility as a proxy for reproductive fitness. Detrimental effects of pH and temperature may only be uncovered when these factors are examined together, or conversely, negative impacts of one variable may be buffered by changes in another. Our results raise the intriguing possibility that some species may cope better with ocean acidification if they simultaneously experience ocean warming.